As a result of the energy crisis, there has been a considerable interest in developing alternative sources of energy for electric power stations to replace increasingly expensive imported petroleum products. One method which has generated considerable interest is the use of solar energy as a heat source for generating steam for electric power generation. In this process, it is necessary to concentrate the energy coming from the sun to heat a liquid which absorbs the heat and carries it to the generator. In relatively low power hot water or space heating systems, the liquid is water. This is either pumped or fed by gravity through a plurality of solar panels to a storage medium, generally crushed rock, from which the heat is extracted for subsequent use. Such systems are beginning to find wide use in home heating and hot water applications, particularly in the Southwest where long periods of sunlight are the norm. However, when it is desired to use solar energy as a source for large scale electric power application, the relatively low boiling point of water makes its use unsatisfactory unless high pressure components are used throughout the system. It would be much better if a low pressure system could be used.
One type of thermal transfer medium which has evoked considerable interest is mixed inorganic salts having a relatively large spread between their melting and boiling points. Many of these have relatively high specific heats so that they are able to absorb large quantities of thermal energy per degree of temperature rise more efficiently than many types of metallic or organic heat transfer media. When liquified, these salts can be readily pumped to and through a "power tower" or similar heat concentration system to obtain a large quantity of sensible heat which is then utilized in a more-or-less conventional steam generator/turbine system for electric power generation. For such use, a number of salt mixtures are known which are stable at temperatures of up to about 900.degree. F.
One particular salt mixture which has been proposed as a practical thermal storage medium for these purposes is a eutectic mixture of sodium nitrate and potassium nitrate. Such a mixture meets the above-stated criteria in that its melting point is relatively low, being about 400.degree. F. and, when molten, is both quite fluid and, most critically, stable at temperatures of up to about 1200.degree. F. Furthermore, the eutectic composition, unlike those of most metallic alloys, is quite broad, ranging from about 30% to about 70% by weight of sodium nitrate in the mix. Thus, tolerance to minor changes in the sodium/potassium ratio is extremely high. Further, the raw materials are widely available and relatively low in cost.
However, to avoid potential operating problems with stress corrosion and line pluggage, it is necessary that both components of the mix be essentially free of chloride and high melting point carbonate, oxide and sulfate impurities, with a combined maximum total purity level of about 0.7% being considered acceptable and with the chloride level being no more than about 0.2%. Further, once having produced the material to this level of purity, there remains the problem of keeping it substantially at such a level once the material is put in use.
In common with power systems such as pressurized water nuclear reactors, a solar power generating system is a dual loop system. In this, the heat generation aspects are in one loop and the power generation aspects in a second loop. For all practical purposes, these loops are completely separate, being joined only at a heat exchanger type steam or vapor generator wherein the working fluid for the power turbine is formed. Because of the hazards associated with exposing a molten oxidizing medium at high temperature, the heat generation loop is kept closed with the cooled but still fluid nitrate salt mixture being pumped either back to the power station heliostat field for reheating or to an internal reservoir for storage until needed.
There is relatively little experience in using mixed nitrate salts in closed loop heat transfer systems. The largest reported use was with reactors employing the Houdry fixed bed process for cracking petroleum. These were mainly used from about 1940 to about 1950 during which time it was found that the molten salt would react with water vapor or carbon dioxide to form respectively hydroxide or carbonate contaminants. Even small amounts of these tended to increase the corrosive nature and, more significantly, raise the melting point of the fluid. Further, it was found that the carbonates tended to form a tightly adhering scale in areas of low velocity flow thus increasing the pressure drop within the system.
Mixed nitrate salt for power station use is provided in the form of dry, non-friable flakes or granules having a melting point of about 400.degree. F. The nominal composition is about 60 weight % NaNO.sub.3 and about 40 weight % KNO.sub.3. Current published product specifications state the minimum acceptable purity to be about 99.3% with the maximum allowable combined carbonate and hydroxide content being about 0.4%. The salt is charged into a melting chamber where it is heated to a temperature of between about 450.degree. and 600.degree. F. and preferably to between about 500.degree. and about 550.degree. F. under an inert atmosphere. When the charge is fully melted, it is then ready for use within the system.
In use, the molten salt will react with traces of water vapor and carbon dioxide to form hydroxides and carbonates along with some oxides of nitrogen. Further, if the temperature at local hot spots in the system exceeds about 1250.degree. F., the nitrates will break down to form alkali metal oxides, oxygen and nitrogen. The oxides, in turn, will react with water and CO.sub.2 according to the equations: EQU M.sub.2 O+H.sub.2 O.fwdarw.2MOH (1) EQU M.sub.2 O+CO.sub.2 .fwdarw.M.sub.2 CO.sub.3 ( 2)
Where M is Na or K
While proper design of the system will keep the levels of these contaminants to a minimum, they cannot be completely eliminated. Therefore, it is necessary to remove them, on a more or less continuous basis, within the confines of the closed nitrate salt loop.